1. Field of the Invention
The present invention relates generally to optical data storage systems. More particularly, the present invention relates to the use of micro-machined elements in optical data storage systems.
2. Background Art
In a magneto-optical storage system, using a magneto-optical (MO) recording material deposited on a rotating disk, information may be recorded on the disk as spatial variations of magnetic domains. During readout, a magnetic domain pattern modulates an optical polarization, and a detection system converts a resulting signal from optical to electronic format.
In one type of a magneto-optical storage system, a magneto-optical head assembly is located on a linear actuator that moves the head along a radial direction of the disk to position the optical head assembly over data tracks during recording and readout. A magnetic coil is placed on a separate assembly on the head assembly to create a magnetic field that has a magnetic component in a direction perpendicular to the disk surface. A vertical magnetization of polarity, opposite to that of the surrounding magnetic material of the disk medium is recorded as a mark indicating zero or a one by first focusing a beam of laser light to form an optical spot on the disk. The optical spot functions to heat the magneto optical material to a temperature near or above a Curie point (a temperature at which the magnetization may be readily altered with an applied magnetic field). A current passed through the magnetic coil orients the spontaneous vertical magnetization either up or down. This orientation process occurs in the region of the optical spot where the temperature is suitably high. The orientation of the magnetization mark is preserved after the laser beam is removed. The mark is erased or overwritten if it is locally reheated to the Curie point by the laser beam during a time the magnetic coil creates a magnetic field in the opposite direction.
Information is read back from a particular mark on the disk by taking advantage of the magnetic Kerr effect so as to detect a Kerr rotation of the optical polarization that is imposed on a reflected beam by the magnetization at the mark of interest. The magnitude of the Kerr rotation is determined by the material""s properties (embodied in the Kerr coefficient). The sense of the rotation is measured by established differential detection schemes and, depending on the direction of the spontaneous magnetization at the mark of interest, is oriented clockwise or counter-clockwise.
Conventional magneto-optical heads, while presently providing access to magneto-optical disks with areal densities on the order of 1 Gigabit/in2, tend to be based on relatively large optical assemblies which make the physical size and mass of the head rather bulky (typically 3-15 mm in a dimension). Consequently, the speed at which prior art magneto-optical heads are mechanically moved to access new data tracks on a magneto-optical storage disk is slow. Additionally, the physical size of the prior art magneto-optical heads limits the spacing between magneto-optical disks. Because the volume available in standard height disk drives is limited, magneto-optical disk drives have, thus, not been available as high capacity commercial products. For example, a commercial magneto-optical storage device presently available provides access to only one side of a 130 mm double sided 2.6 ISO gigabyte magneto-optical disk, a 40 ms disk access time, and a data transfer rate of 4.6 MB/Sec.
N. Yamada (U.S. Pat. No. 5,255,260) discloses a low-profile flying optical head for accessing an upper and lower surface of a plurality of optical disks. The flying optical head disclosed by Yamada describes an actuating arm that has a static (fixed relative to the arm) mirror or prism mounted thereon, for delivering light to and receiving light from a phase-change optical disk. While the static optics described by Yamada provides access to both surfaces of a plurality of phase-change optical disks contained within a fixed volume, Yamada is limited by how small the optics can be made. Consequently, the number of optical disks that can be manufactured to function within a given volume is also limited. Another shortcoming relates to the use of static optics. This approach imposes a limit on track servo bandwidth by requiring the entire optical head assembly to move in order to change the location of a focused optical spot. This same limitation applies to the flying magneto-optical head disclosed by Murakami et aL in U.S. Pat. No. 5,197,050. In general, the larger the mass of the element used to perform fine track servoing, the lower the servo bandwidth becomes and the lower the track density that can be read or written.
A method for moving a folding prism or mirror with a galvanometer actuator for fine tracking has been disclosed by C. Wang in U.S. Pat. No. 5,243,241. The galvanometer consists of bulky wire coils and a rotatable magnet mounted on a linear actuator arm attached to a flying magneto-optical head, but not mounted on the slider body itself. This design limits the tracking servo bandwidth and achievable track density due to its size and weight. Its complexity also increases the cost and difficulty of manufacture.
What is needed is an improved optical head that is compact and that allows an increase in the number of disks that can be placed within a given volume as compared to the prior art. The improved optical head should preferably provide a high numerical aperture, a reduced head size and mass, and a high resonance frequency tracking servo device that provides a very fine track servo bandwidth. Additionally, the optical head should improve upon prior art access to disk surfaces, disk drive access times, data transfer rates, and ease of alignment and manufacture.
The present invention provides improvements over prior art optical disk drives. The improvements allow an increase in the number of storage disks that can be placed within any given volume. The improvements enable the use of a high resonance frequency tracking servo device on a reduced profile head to provide improved access to storage media, improved disk drive access times, and improved data transfer rates.
The optical disk of the present invention utilizes Winchester magnetic disk technology. A laser optics assembly couples an optical light source through a small micro-machined optical switch to one or more rotary arms, each of which support an optical head for writing and reading data to the storage media. Lighting is delivered through an optical fiber to a respective optical head for the purpose of probing the storage media with a focused optical spot. The reflected light signal from the storage media then couples back through the optical head for processing.
The light transmitted from the optical fiber to the optical head is affected by a micro-machined element. In the preferred embodiment, the light is affected by a steerable micro-machined mirror. Track following and seeks to adjacent tracks are performed by rotating a central mirror portion of the steerable micro-machined mirror about an axis of rotation. A reflected light from the steerable micro-machined mirror is directed through an embedded micro-objective lens such as a GRIN (Graded Index) lens or a molded lens. A focused optical spot is scanned back and forth in a direction which is approximately parallel to the radial direction of the storage media. In a second preferred embodiment, track following and seeks to adjacent tracks may be performed with more than one storage media at a time by operating a set of steerable micro-machined mirrors independently from each other.
The steerable micro-machined mirror includes a flexure layer having a structure defining an opening. A central mirror portion is disposed in the opening. The central mirror portion includes a parallelogrammatic reflective structure that includes a pair of first opposed sides and a pair of second opposed sides, with the pair of flexure layer hinges being integrally bound to the pair of the first opposed sides and to the flexure layer. In another preferred embodiment, at least one tether member may be integrally bound to the second opposed sides of the central mirror portion and to the flexure layer. The at least one tether member includes a structure defining at least one tether channel. The tether functions for limiting a range of movement of the mirror and for preventing the mirror from contacting an actuation electrode. In another preferred embodiment, the steerable micro-machined mirror includes: a substrate, at least one actuation electrode supported by the substrate, and at least one plate member supported by the at least one actuation electrode. The actuation electrode may include a first electrode surface, and a second electrode surface which is generally parallel to the first electrode surface and at a different elevation than the first electrode surface.
In a preferred embodiment, the steerable micro-machined mirror is attached to a flying magneto-optical head. The flying magneto-optical head is preferably one of a set of magneto-optical heads for use in a magneto-optical system.